A microstrip antenna is one of antennas used in in-vehicle millimeter-wave radars. The microstrip antenna can be manufactured by a process of etching a conductor film on an insulating substrate, and therefore downsizing and cost reduction thereof can be achieved.
Generally, antennas used in in-vehicle millimeter-wave radars have a smaller vertical-plane beam width to only focus on target objects on the road, while having a larger horizontal-plane beam width so as to provide a wide detection range.
As the horizontal-plane beam width increases, the antenna is likely to be affected by diffracted waves generated at edges of a ground conductor. When the diffracted waves interfere with an original received wave, a phase difference periodically varies with different horizontal angles of the beam. This variation causes ripples in an antenna gain and a phase radiation pattern, which degrades detection performances of the radar, such as angle measurement accuracy.
As a method for solving these problems, there has been a method of reducing diffracted waves by applying or attaching a radio wave absorber on edges of an antenna substrate.
In addition, Patent Literature 1 discloses a technique in which a passive element is provided on edges of an antenna substrate in a microstrip antenna, and the passive element has an electrical width of one-half of a wavelength of transmitted or received electromagnetic waves, so that diffracted waves generated from a radiation element side and diffracted waves radiated from an antenna substrate edge-side portion cancel out their phases with each other.